The present invention relates generally to planar optical waveguide devices and particularly to planar optical waveguide devices in which at least one coating layer is provided which has a coefficient of thermal expansion sufficient to impart tensile stress to the device in response to a change in temperature.
It has been observed that optical waveguide devices made of polymeric materials can exhibit differences in response characteristics depending upon the ambient temperature. It would be useful if the rate of change with temperature of planar optical waveguide response characteristics could be controlled. This would provide optical devices whose waveguide responses may be selected which are either substantially unaffected by minor temperature fluctuations or are materially affected by such changes (i.e. selected for their tuning capability). As used herein the term xe2x80x9ctuningxe2x80x9d means that the optical filter element of the optical signal device may have its ability to reflect light varied preferentially for a preselected wavelength.
For example, gratings made in planar polymeric optical waveguides can exhibit significant changes in spectral response as temperatures vary. If such changes are not desirable it is necessary to fabricate multiple waveguide devices depending on the range of operating temperatures. In some cases it is desirable that little or no change in spectral response occurs with temperature. For instance, if a grating is designed to preferentially isolate a particular frequency from a host of other frequencies, it is desirable that the selected frequency does not change throughout the typical ambient temperature fluctuations that occur during use. Such temperature fluctuations can adversely affect the accuracy of readings, or require highly accurate forms of temperature control.
In other instances, however, it is desirable that the change in spectral response occur at a controlled rate (i.e., the device has tuning capability). For example, an optical sensor comprising a waveguide and a grating could be used to measure temperature. The sensitivity of the sensor would be related to the ability to control the rate at which the wavelength response varies with temperature (i.e., the control of dxcexB/dT).
The sensitivity (dxcexB/dT) for a planar polymeric grating of an optical signal device is currently determined by the intrinsic properties of the planar waveguide materials. These intrinsic properties include the coefficient of the thermal expansion (CTE) and the change in refractive index of the materials with temperature (dn/dT). The CTE and dn/dT properties vary linearly with temperature and their values are dependent on the composition of the materials used to fabricate the optical waveguide devices. Finding suitable materials to make gratings in planar optical waveguides with the required optical properties can be extremely difficult. To then require that the material have the correct temperature sensitive response (dxcexB/dT) can make the problem of materials selection even more difficult. It would, therefore, be beneficial to have an optical signal device in which dxcexB/dT could be either set to zero or controlled within a desirable range without having to change the composition of the waveguide materials. This would enable a single waveguide device to operate within a range of selected values for dxcexB/dT.
Such control would also be beneficial in the use of planar optical directional couplers. Single mode optical directional couplers are normally used as interferometric beam splitters to split signals into numerous alternative paths. Most commonly they are input/output devices where one input is split among two outputs with some characteristic splitting ratio. This splitting ratio is affected by slight dimensional changes in the spacing between the optical paths. By controlling either of the operative components of the optical signal device (CTE and/or dn/dT), it would be possible to control the sensitivity of the splitting ratio.
Moreover, the use of operative components with a controlled CTE could also have use in the control of multi mode Interference (MMI) devices. Such devices are strongly dependent on their dimensions and can, therefore, be affected by temperature fluctuations. Proper selection of components with controlled CTE could help control their performance as well.
It would therefore be a significant advance in the art of producing and using optical signal devices to provide such devices with a controllable sensitivity (i.e. to control the rate at which the frequency response varies with temperature).
It would be a further advance in the art to produce and use optical signal devices in which the device has tuning capability.
The present invention is generally directed to an optical signal device having controlled sensitivity particularly to fluctuations in temperature. In a particular aspect of the present invention there is provided.
An optical signal device comprising:
a) a planar polymeric optical signal device which is temperature sensitive and having waveguide layers therein, and
b) at least one material incorporated into said optical signal device, having a co-efficient of thermal expansion of from about 20 to 200 ppm/xc2x0K sufficient to impart tensile stress to said waveguide layers as the temperature of the optical signal device changes.
In a particular aspect of the present invention, the material is incorporated as at least one separate layer or forms at least a portion of a substrate.